Heidi Klum believes in fairy tales -- and feels "horrible" that her seven-year marriage with
Seal didn't have a happily ever after.

Appearing today (Sept. 12) on
Katie Couric's new talk show,
"Katie," the supermodel opened up about their very public split -- and her relationship with bodyguard Martin Kirsten.

"It's not something that I wanted, you know," the mother of four tells Couric. "I'm someone who believes in a fairy tale, and when I said 'I do'...I meant forever.

"But life changes -- it's that road with all those bumps and holes and you trying to struggle through them and -- I don't know, we just couldn't make it work."

"I tried my hardest, you know he tried his hardest, [but now] I want to move forward," Klum continues. "I want to be friends with him, I want us to be there for our children. And it's horrible -- we both didn't want that."

As for her relationship with the family's bodyguard,
Martin Kirsten, Klum says, "He's a great man, and recently we just got to know each other from a completely different side.

"He's been with our family for the last four years. He's cared for our four children and helped us tremendously. I trust him with my children's life."